The Oxford English Dictionary also lists only the verb and the noun, but under the noun it has various compounds:
a. Forming compounds denoting a further example or instance of the specified noun, as repeat business, repeat order, repeat performance, etc.
b. Forming compounds denoting a person who does something (implied by the second element) again or repeatedly, as repeat customer, repeat offender, repeat viewer, repeat visitor, etc.
Linguists do not generally regard a modifier like "repeat" in "repeat performance" as an adjective, because it doesn't behave like an adjective in other ways (eg "*The performance was repeat", "*A very repeat performance", "*The repeatest performance")
Best Answer
What you have there is a noun adjunct.